Many applications require that a high current is driven to an external device. A high-current driver may be used to drive a relatively large current to a Light-Emitting Diode (LED), liquid crystal display (LCD), motor, actuator, etc. Health-care applications requiring a large current include heart-rate monitoring, SPO2 monitors, and other sensors.
FIG. 1A shows a high-current application. Current source 106 provides a large current that is switched by switch 104 to turn on and off LED 102. FIG. 1B is a waveform of the high-current switching application. The switch closes when switch voltage VSW is high. The source current switched through LED 102 pulses high when the switch is closed. Some low-power applications may save power by reducing the turn-on time. A large current may be needed to provide a rapid settling time for the source current so that the waveform is not distorted.
As semiconductor process technology improves, devices sizes shrink. These smaller devices use reduced power-supply voltages to prevent damage to the tiny devices. The lowered power-supply voltage produces a low-headroom environment for the circuit where smaller voltages are applied across transistors. The smaller voltages in a low-headroom environment produce lower currents, which is opposite of the design goal for a high-current driver circuit. Therefore careful circuit design is needed.
A traditional current driver might use a complementary metal-oxide-semiconductor (CMOS) current mirror. However, the output current varies with changes in the drain-source voltage. Cascode current mirror may be used to reduce the current variation with drain-source output voltage, but a large voltage drop occurs on a cascode transistor. In low-headroom environments, there may be insufficient available voltage drop for the cascode transistor.
When a second transistor is placed in series with a large output transistor, this second transistor also has to be large to carry the large current, increasing circuit size and cost. Also the second transistor may reduce the available voltage drop to the output transistors. Thus having one or more transistors in series with the output transistor is undesirable.
Many driver circuits use operational amplifiers (op amps). Op amps provide a quick response, but have a high gain and have high power consumption. It is thus desirable to avoid op amps in a high-current low-headroom driver circuit.
Other circuits use a resistor over a constant voltage to generate the constant current. However, the resistor's voltage drop reduces the available voltage for other transistors in the circuit, and the resistor burns power.
What is desired is a low-headroom high-current driver circuit. A current driver circuit that does not use an op amp is desirable. A current driver circuit that does not use a resistor to generate a constant current is also desirable. A driver circuit with a constant current mirror source for high-current applications in a low-headroom environment is desired.